Since its launch in February 2006 AKARI, a Japan Aerospace Exploration Agency (JAXA) mission with ESA participation, has been working flawlessly and has already produced outstanding views of the infrared Universe. New results, to be presented this week at the annual meeting of the National Astronomical Society of Japan, provide unprecedented glimpses of regions of intense star formation, views of stars at the very end of their life, supernova remnants never detected before in the infrared, distant galaxies and active galactic nuclei harbouring black holes surrounded by clouds of molecular gas.
This colour composite image represents the very first infrared observation of a supernova remnant (called B0104-72.3) in the Small Magellanic Cloud, a small galaxy neighbouring the Milky Way. It is visible from the southern hemisphere and it is located at a distance of 200 000 light years from the Earth. The object was observed by AKARI’s Near- and Mid-Infrared Camera (IRC) at wavelengths of 3, 4, 7 and 11 micrometres. The pair of arc-like structures forming the spatial extent of the supernova remnant cover approximately 60x100 light years and the white bar in the bottom-left corner indicates a length of 30 light years. In the final stage in the life of stars much heavier than our own Sun, a supernova explosion ejects matter into interstellar space, creating a supernova remnant. The matter ejected consists of newly synthesized elements created inside the star’s core. The catastrophic explosion disturbs interstellar material creating shock waves over great distances, thus making supernovae essential contributors in the internal evolution of galaxies. B0104-72.3 is a well-known supernova remnant that has been observed with radio and X-rays, (although not particularly bright at these wavelengths). Infrared observations help trace the interaction between the supernova remnant and the interstellar medium, but this understanding has until recently been limited due to the lack of data. B0104-72.3 was observed by AKARI at four different infrared wavelengths, the intensity ratios of which reveal the presence of a shock at the interaction between the expanding remnant and the surrounding molecular clouds. This implies that the precursor was a high-mass star. In this manner, observations with AKARI are expected to play a key role in understanding the evolution of interstellar material through studies of interaction processes with the supernova remnant. Credits: JAXA
This image shows star formation revealed by AKARI’s wide-area survey in the reflection nebula IC4954/4955. The nebula is located in the constellation Vulpecula, around 6500 light years from us. The Near- and Mid-infrared Camera (IRC) and the Far-Infrared Surveyor (FIS) instruments onboard AKARI carried out observations of this region at seven different infrared wavelengths and revealed a continuing cycle of star formation over three generations, across enormous spatial scales (the actual scale of the picture is approximately 13x20 light years). Astronomers believe that these high density molecular clouds located in IC4954/4955 often participate in star formation when supernova explosions and intense radiation from young high-mass stars sweeps the interstellar material together into these high-density regions. AKARI infrared observations over the wavelength range from 9 to 160 micrometres detected the evidence of such star formation (marked by green crosses). The image is a three-colour composite image from AKARI’s 9 (blue), 11 (green), and 18 (red) micrometre data. The two arc-like structures visible in the panel are formed by a young-massive star (not seen in the image) at the centre of the nebula, sweeping material outward by their strong radiation pressure. Credits: JAXA
When it concerns studying the formation and evolution of stars, and in more general, the evolution of galaxies in the Universe, infrared satellites like AKARI have a clear advantage. The matter ejected into interstellar space from old stars is warmed up by the stellar radiation from younger stars and by collisions with the material already present in interstellar space, and re-emits this energy at infrared wavelengths.
Since young stars are formed in high-density regions, where the interstellar gas and dust is thickest, the surrounding material veils the light from the star making observations with normal visible light extremely difficult and sometimes impossible. The absorbed light from the central shrouded star is also re-emitted at infrared wavelength. This is also the case for distant galaxies, especially newly born ones, which (like the young stars they contain) are thought to be covered by thick interstellar material. So, infrared observations play a truly crucial role for our understanding of such ‘dusty’ targets, invisible or barely visible when observed at other wavelengths.
From stellar nurseries to dying stars
The first set of AKARI results presented in this article focuses on the evolution of interstellar material in galaxies, including the cycle of star formation, supernova remnants – the residual remaining from massive stars which end their life through a catatrophic explosion - and mass-loss from red-giant stars.New stars are born in the densest regions of interstellar clouds of gas and dust, and among the many complicated triggers for star-formation, compression of interstellar clouds by supernova explosion shocks and the strong radiation emitted from high-mass stars are especially important. Stars born in such clouds will live out their lives and also eventually evolve into supernovae or red-giants, thus providing the material for the next generation star formation and so on.
They have in fact extinguished their stock of fuel (hydrogen at their core) and evolved into red-giant stars. Through its observations, AKARI provided the first ever evidence for high-rate mass loss from the young red-giants in this cluster, a phenomenon thus far only observed in older red-giants.From giant black-hole candidates to new-born galaxies
Following on from the studies initiated by ESA's Infrared Space Observatory (ISO), AKARI detected 280 galaxies of this kind, performing the deepest ever wide-area survey of the Universe at this wavelength and confirming the intense star and galaxy formation in that early period of our cosmic history.AKARI is also performing similar studies over a wide range of wavelengths – a study that will provide a definitive description of the galaxy evolution over the entire lifetime of our Universe.
AKARI is a JAXA's mission carried out with the participation of several partners such as: the Nagoya University, The University of Tokyo and National Astronomical Observatory Japan; the European Space Agency (ESA); the Imperial College London, the University of Sussex and The Open University (UK); the University of Groningen / SRON (The Netherlands); the Seoul National University (Korea). The far-infrared detectors were developed under collaboration with The National Institute of Information and Communications Technology (Japan).ESA/ESAC provides expertise and support for the sky-survey data processing through the pointing reconstruction – this allows the determination of accurate astronomical positions for each of the sources detected. ESAC also provides user support for the European astronomers who have been granted observing opportunities. ESA/ESOC is providing the mission with ground support through its ground station in Kiruna, for several passes per day.
Alberto Salama | alfa
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